chos mngon pa kun las btus pa - Sanskrit Title 1

The text then continues with this standard set of parts. If you learn these, you should be able to translate and understand most if not all of the Kangyur and Tengyur texts translated to tibetan.
The first part is rgya gar skad du. rgya gar is India but note later that there will be a twist to this! skad is voice or language. du is a general sub-ordination particle that will classify what is to the right with what is on the left. Use the word in here.

So the first pass on translating this is In the language of India. However, as the Mahayana Buddhist texts were written in Sanskrit, you might as well translate this as: In Sanskrit.

Next, how to learn how the Tibetans transcribed Sanskrit words using Tibetan letters.